Helical-walled waveguides



9, 1966 G. COMTE 3,289,121

HELICALWALLED WAVEGUIDES Filed June 16, 1964 //v VE/VTOR (15026153 Comm m Q1 9 72. 0, Q.

United States Patent Patented Nov. as, less Free Helical-walled circular waveguides are known which comprise a dielectric substance which is disposed between the helical wall and a screen consisting of a tube of metal fabric, the assembly being covered by an external protective sheathing. Such guides effect the transmission of Waves of the TE type, while producing a considerable attenuation of other types of waves.

It has been observed, both theoretically and experimentally that this attentuation is maximum when the distance between the metal screen and the external surface of the helical wire is equal to a quarter radial wavelength in the interposed dielectric substance. It follows that the attenuation to which waves of undesirable types are subjected by such a waveguide is maximum for a given frequency and decreases on either side of the latter which leaves only a narrow useful frequency band. However, it has been shown that it is possible to extend the filtering band of such guides by embedding in the dielectric substance, within the aforesaid screen, one or more semireflecting layers, but the useful band width remains limited to less than one octave, which may cause difficulties in extending the use of circular waveguides in telecommunications.

The present invention relates to a helical waveguide of which the filtering effect on parasitic modes retains a a maximum value in a very wide frequency range, which may reach a number of octaves, without the quality of this filtering being inferior to that obtained under the best conditions by previously known means. Such a result can be obtained only if the dielectric medium disposed between the metallic screen and the outer surface of the helical wire always constitutes, for electromagnetic waves excited by the undesirable modes, an electrical path equal to one quarter of a radial wavelength A, or to an odd multiple of A A, in the material under consideration, regardless of the frequency employed. This may be achieved only if the radial dielectric constant e,- of the interposed material satisfies the condition d denoting the thickness of this material, K being an integer and A denoting the wavelength in vacuo at the frequency 1 under consideration.

This condition may be written:

= constant=d tric properties, so chosen that the relaxation phenomena of the molecules of the dielectric occur in the frequency band in which the guide is used. It is known that the effective dielectric constant e' of such materials may be placed in the form e s+ 0 2 1m) oo denoting the dielectric constant at infinite frequency (corresponding, for example, to visible light), 6 the static dielectric constant (measured at very low frequency), t the relaxation time of the material under consideration, s the dielectric constant of a vacuum and w the pulsation=21rf at the frequency under consideration.

It is found that if the frequency is suffioiently high to have and if the material has a sufficiently high static dielectric constant 6 this relation is reduced substantially to:

,1 s'i 0 2 2 w t e co +2 0 It is then suflicient to choose the thickness d in such manner that the desired relation is satisfied, by writing:

By way of non-limiting example, the material incorporated in the filling dielectric is chosen from the plumbates, the titanates, the niobates, the tantalates, the tungstates, etc., of alkaline-earth metals, all of which substances have very pronounced ferroelectric properties, while the relaxation phenomena are generally manifested in the centimetric or the millimetric wave range, which are precise ly utilised for transmission by circular waveguides. However, some organic substances having polar molecules, such as aluminium-g uanidine sulphate, may also constitute dielectrics having appropriate characteristics, because the relaxation phenomenon is extremely widespread.

The substances under consideration may be incorporated in the filling dielectric in any way, for example by mixing with a liquid thermosetting resin, by coating insulating tapes with a solution or suspension of fer-roelectric substances, or by any other known method.

The accompanying drawing shows, by way of example, a longitudinal cross-section of a waveguide according to the invention. In this drawing, 1 designates the helically coiled metallic wire constituting the internal conductor of the guide, 2 indicates the dielectric layer in which is incorporated a material having polar or ferroelectric molecules, 3 indicates the metallic fabric screen, 4 an insulating layer and 5 an external protective sheath.

Iclaim:

1. Circular waveguide having a helical wall, comprising a dielectric substance between the said wall and an external metallic screen, characterised in that there is incorporated in the dielectric substance an insulating material comprising polar or ferroelectric molecules which 3 has a relaxation effect in the frequency band transmitted by the guide such that the electrical thickness of the dielectric substance is constant over substantially the entire frequency band.

2. Waveguide according to claim 1, characterised in that the incorporated insulating material is chosen from the plumbates, the titanates, the niobates, the tungstates, the tantalates of alkaline-earth metals.

3. Waveguide according to claim 1, characterised in that the incorporated insulating material is an organic material such as hydrated aluminium-guanidine sulphate.

References Cited by the Examiner UNITED STATES PATENTS 2,950,454 8/1960 Unger 33395 HERMAN KARL SAALBACH, Primary Examiner.

L. ALLAHUT, Assistant Examiner. 

1. CIRCULAR WAVEGUIDE HAVING A HELICAL WALL, COMPRISING A DIELECTRIC SUBSTANCE BETWEEN THE SAID WALL AND AN EXTERNAL METALLIC SCREEN, CHARACTERIZED IN THAT THERE IS INCORPORATED IN THE DIELECTRIC SUBSTANCES AN INSULATING MATERIAL COMPRISING POLAR OR FERROELECTRIC MOLECULES WHICH HAS A RELAXATION EFFECT IN THE FREQUENCY BAND TRANSMITTED BY THE GUIDE SUCH THAT THE ELECTRICAL THICKNESS OF THE DIELECTRIC SUBSTANCE IS CONSTANT OVE SUBSTANTIALLY THE ENTIRE FREQUENCY BAND. 